Multilayer polymeric films

ABSTRACT

The use of multilayer film structure comprising at least two unfilled layers of polymeric material substantially devoid of opacifying agent and at least two filled layers of polymeric material wherein said filled layers comprise at least 5% by weight of opacifying agent for providing improved tear-resistance in an opaque polymeric film, particularly a polyester film.

The present invention relates to opaque multilayer polymeric films.

Opaque polymeric films are well known in the art and have a wide varietyof uses, including use generally as a paper replacement (such as infilms for graphics, displays, labels, cards (including identity cards,smart cards and credit cards) and imaging), as printing plate media, inpackaging and in coil coating. Opaque films are generally prepared bythe incorporation of fillers and pigments, especially white pigments,into a film-forming polymer.

The cost of manufacturing polmeric film is influenced by severalfactors, including the amount and price of die additive, the technicalcomplexity of incorporating the additive into the film, and theefficiency of the manufacturing process. The cost of manufacture ofopaque film is naturally greater than the cost of manufacture ofunfilled film because of the cost of the additional components. Thus,although the incorporation of greater amounts of filler proportionallyincreases the opacity of a film, it is normal in conventionalmanufacturing processes for a compromise or balance between cost andopacity to be reached.

Besides the increased cost, there is a further disadvantage to theincorporation of fillers. Although being a convenient means formodulating the light transmission properties of a film, theincorporation of fillers can have a detrimental effect on the mechanicalproperties of the film. In particular, the incorporation of filler canadversely affect the tear-resistant properties of the film. A hightear-resistance is advantageous during the manufacturing process inorder to improve economy, efficiency and productivity. A hightear-resistance is also advantageous in the general handling of the filmand in the end use of the film.

In conventional manufacturing processes, a greater film thickness hasbeen used in order to compensate for the adverse affect on mechanicalproperties caused by the use of filler. A greater film thickness, inrelation to a transparent film having little or no filler is alsorequired in order to incorporate sufficient filler to achieve thenecessary opacity. A thicker film requires the use of greater amounts offilm-forming polymer which, again, increases the cost of manufacture Athinner gauge film which retains the requisite opacity and whichexhibits good tear-resistance would therefore be economically desirablefor price-sensitive markets

Tear-resistant films are known In the art. EP-0592284 discloses atear-resistant multilayer film comprising alternating layers of stiffand ductile polymeric materials which may be useful as a laminate forshatter-proofing a glazing member. U.S. Pat. No. 5,759,467 discloses amultilayer polyester film which comprises a plurality of alternatinglayers of terephthalic acid-based polyester and naphthalene dicarboxylicacid-based polyester. The film has increased tensile strength and isstated as being of use in, inter alia, magnetic media substrates.

The preparation of multilayer films may be achieved in a variety ofways. U.S. Pat. No. 3,647,612 discloses a process for the preparation ofa multilayer film which involves providing two or more streams ofthermoplastic material, arranging the two or more streams into a singlestun having a plurality of generally parallel layers, mechanicallymanipulating the stream by dividing and recombining to provide anincreased number of layers and then forming the steam into a thin sheetor film. The layers are composed of resinous material which istransparent to visible light, the multilayer film structure having aniridescent appearance. EP0426636 also discloses multilayer coextrudedlight-reflecting films comprising a plurality of generally parallellayers of transparent thermoplastic resinous material.

EP-0492894 discloses a method and apparatus for the production of amultilayer film by generating interfacial surfaces in a fluid mass. Themethod comprises the steps of dividing a first composite stream into atleast two branch streams, repositioning the branch streams, expandingsymmetrically along one axis, contracting symmetrically along anotheraxis and recombining the branch streams into a second composite streamwhich comprises a greater number of discrete layers of polymericmaterial than the first composite stream, wherein the expansion andcontraction steps are conducted either on the individual branch streamsor on the second composite stream. WO-98/06587 discloses a polyesterfilm having an opaque, preferably black, core layer having an opticaldensity greater than 2.0 and on both surfaces thereof a white outerlayer for use as a photographic sheet or other imaging applications.

EP-A0933199 discloses an opaque multilayer film having at least onepolyester layer containing more than 5 wt % of a pigmient, and at leastone polyester layer substantially devoid of pigment, wherein the ratioof the respective thicknesses of the layer(s) devoid of pigment to thepigmented layer(s) is at least 1. The film is stated as minimizing theoverall pigment content while providing high opacity and good mechanicalproperties (modulus, tensile strength and force at 3% elongation, 5%elongation and elongation at break).

It is a general object of the invention to influence and improve thebalance between the efficiency of manufacture of the film and thedesired properties of the film. Thus, one object of the invention is toreduce manufacturing costs while maintaining or improving the desiredproperties of the film. A further object of the invention is to enhancethe performance of the film without increasing manufacturing costs. Inparticular, it is an object of this invention to provide an opaque filmwhich is economical to manufacture and which has improvedtear-resistance. It is a further object of this invention to reduce thethickness of an opaque film while retaining the requisite opacity andalso while retaining or improving tear-resistance. It is a furtherobject of the invention to provide a film having an increased opacityfor a given thickness whilst retaining or improving tear-resistance andtherefore manufacturing efficiency.

According to the present invention there is provided the use of amultilayer substrate or structure comprising at least two unfilledlayers of polymeric material substantially devoid of opacifying agentand at least two filled layers of polymeric material wherein said filledlayers comprise at least 5% by weight of opacifying agent for providingimproved tear-resistance in an opaque highly, filled polymeric film.

According to a further aspect of the present invention, there isprovided an opaque multilayer film comprising at least two unfilledlayers of polyester material substantially devoid of opacifying agentand at least two filled layers of polymeric material wherein said filledlayers comprise at least 5% by weight of opacifying agent.

For brevity, the term “an unfilled layer” will be used herein to referto a layer of polymeric material substantially devoid of opacifyingagent (or filler) and the term “a filled layer” will be used herein torefer to a layer of polymeric material comprising at least 5% by weightof an opacifying agent.

As used herein, a reference to a % weight of filler is unless otherwisespecified a reference to the weight of filler relative to the totalweight of the film or relative to the total weight of a given layer(i.e. the weight of the film- or layer-forming polymeric material plusfiller).

For a given opacity, a multilayer film as described herein allows areduction in the amount of filler in the film as a whole relative to afilm which does not have the multilayer structure Thus, for a givenopacity, a multilayer film as described herein allows a reduction in theamount of filler relative to a filled mono-layer film. In general,different fillers produce different degrees of opacity for a givenconcentration of filler.

The film described herein exhibits good opacity and unexpectedlyimproved tear-resistance. The film allows the use of less filler whileretaining the opacity of conventional opaque films and is moreeconomical to produce. The unexpected benefit in tear-resistance meansthat:

-   -   (i) for a given opacity and tear-resistance, thinner films can        be produced; or    -   (ii) for a given opacity and film thickness, a film having an        improved tear-resistance can be produced, with an increased        efficiency of manufacture; or    -   (iii) for a given thickness and tear-resistance, the opacity of        a film can be increased, i.e. the amount of filler in the film        and therefore the opacity can be increased without decreasing        the tear-resistance of the film or adversely affecting the        efficiency of manufacture.

In one embodiment, a multilayer film preferably composes, in the film asa whole, filler in an amount less than about 90%, preferably less thanabout 80%, preferably less than about 70% and preferably less than about60% of the amount of a filler in a monolayer film of equal opacity. Asnoted above, such a film allows film thickness to be reduced for a givenopacity without adversely affecting tear-resistance and manufacturingefficiency. In this embodiment, the overall amount of filler in themultilayer film is preferably less than about 18% filler by weight ofthe film preferably less than about 16%, preferably less than about 12%,preferably less than about 8%, and preferably less than about 4%. Asnoted above, different fillers provide different degrees of opacity. Inthis embodiment, and where the filler is barium sulphate, the overallamount of filler in the film is less than about 18%, preferably lessthan about 16%, and preferably less than 12% by weight of the film.Where the filler is, for example, titanium dioxide or polypropylene theoverall amount of filler is less than about 16%, preferably less than12%, preferably less than 8%, and preferably less than 4%.

In an alternative embodiment, a multilayer film comprises, in the filmas a whole, filler in an amount of at least 110%, preferably at least120%, preferably at least 130%, and preferably at least 140% of theamount of filler in a monolayer film of equal tear-resistance. As notedabove, such a film allows opacity to be increased for a given filmthickness without adversely affecting the tear-resistance andmanufacturing efficiency. In this embodiment, the overall amount offiller in the multilayer film may be more than about 15%, preferablymore than about 20%, preferably more than about 22% and preferably morethan about 25% by weight of the film. For barium sulphate filler, forexample, the overall amount of filler in the film may be greater than22% and preferably 25% or above.

The opacifying agent is present in the filled layer at a level of atleast 5%, preferably at least 10%, and more preferably at least 15%, byweight of the polymeric material of the filled layer. Preferably, theamount of opacifying agent in the filled layer is no more than 30%, andpreferably no more than 25%, by weight of the polymeric material of thefilled layer.

The tear properties of the films described herein are characterisedusing two parameters, namnely the “maximum load” and the “teartoughness”.

The maximum load is a measure of the force required to initiate tearngof the film, i.e. the load at the onset of tearing, and is measured inaccordance with ASTM D1004-94A (Graves Tear Test). This parameter isreferred to in ASTM D1004-94A as the initial tear-resistance, and isexpressed in Newtons or kilograms-force. The maximum load (referred toas initial tear-resistance in the Graves Tear Test) generally increaseswith increasing film thickness. Thus, for the purpose of a faircomparison it is important either that film with similar thickness istested, or that the quantitative relationship between thickness andmaximum load is first determined for the film type under considerationIn this work, data have been collected which shows that maximum loadchanges closely in proportion to thickness and that the Graves Tear Testcan be used reliably to compare filled polyester film whose thicknesslies in the range 50 to 125 μm. Thus, the measurement of maximum load isreported herein after normalisation to a reference thickness of 80 μm.Preferably, the film exhibits a maximum load of at least 3.0 kgf,preferably at least 3.5 kgf and more preferably at least 4.0 kgf at afilm thickness of 80 μm.

The tear toughness is measured as the Graves Area of the film, asdescribed in EP-A-0592284. The Graves Area is obtained by mathematicallyintegrating the area beneath the curve in a graphical plot of the stressversus the strain for a film subjected to the Graves Tear Test (ASTMD1004-94A), i.e. during a test in which a film sample specificallyshaped for a Graves Tear Test is clamped between opposed jaws that aremoved apart at a constant rate to concentrate the tearing stresses in asmall area. The stress is defined as the recorded load divided by theinitial cross-sectional area of the film opposite the notch feature ofthe test sample. The strain is defined as the ratio of the change in thedistance between the jaws (Δl) that occurs during the test, to theinitial separation of the jaws (l), i.e. strain is Δl/l.

Thus, tear toughness may be regarded as a measure of the total energyrequired to cause the film to fail, i.e. the ability of the film toabsorb energy before failure. It will be understood that film with arelatively large tear toughness value will require a larger amount oftotal energy to cause failure, compared to film with a relatively smalltear toughness value. The tear toughness may vary depending on whetherthe test is conducted in the machine or the transverse direction of thefilm.

Preferably, the multilayer films described herein demonstrate a teartoughness in one or both dimensions of the film equal to at least 0.3k/mm², preferably at least 0.6 kg/mm², and more preferably at least 0.9kg/mm².

Preferably the opaque multilayer film comprises at least 5 layers,preferably at least 6 layers and preferably 7 or more layers. The totalnumber of layers in the film is generally limited by the desired filmthickness and by the ease of manufacture. Typically, a film would haveless than 100 layers. In one embodiment, the film has less than 50layers. The number of layers may be adjusted according to the targetthickness of the film and the target opacity. For a given filmthickness, the number of layers will depend on, for instance, thethickness of individual layers and the particle size of the filler. Forinstance, for a 20 μm thick film comprising a filler of an averageparticle size of 1 μm in diameter, and an individual layer thickness of0.5 μm, the number of layers should not exceed about 40.

Preferably the film comprises alternate filled layers and unfilledlayers, i.e. a filled layer is arranged between two unfilled layers andan unfilled layer is arranged between two filled layers.

In a preferred embodiment, the two outermost layers of the film are thesame type of layer, preferably a filled layer. Where the outer layersare of the same type, preferably there is an odd number of layers in thefilm so that each unfilled layer is adjacent to a filled layerthroughout the film.

The film may have, for example, a multi-layer structure (BA)_(D),(BA)_(D)B or (AB)_(n)A wherein A designates an unfilled layer; Bdesignates a filled layer and n is an integer of at least 2, preferablyat least 3, and preferably at least 4. Preferably, the film has astructure (BA)_(D)B, preferably wherein n is at least 3, and preferablyat least 4.

The thickness of each layer and the total thickness of the film may bevaried over wide limits within the scope of the invention. The practicalthickness of the film is limited only by the handling characteristicsdesired. The lower useful practical limit is that at which the filmbecomes too flimsy to be readily handled or is no longer sufficientlytear resistant while the upper useful limit is that at which the filmbecomes overly rigid and too difficult to process.

The total thickness of the multilayer film described herein ispreferably in the range from about 5 μm to about 350 μm, preferably fromabout 5 μm to about 200 μm. In one embodiment, the film thickness ispreferably from about 5 μm to about 100 μm, more preferably from about 5μm to about 50 μm, more preferably from about 5 μm to about 20 μm, andparticularly from about 12 μm to about 20 μm. In an alternativeembodiment, the film thickness is in the range from about 50 μm to about125 μm. The thickness of the individual layers may also vary over a widerange, it being understood that as the number of layers increases at aconstant or decreasing film thickness, the thickness of each layerdeclines. An individual filled layer typically has an average thicknessof at least about 0.1 μm, and in one embodiment at least about 0.5 μm.Although the thickness of each layer may be the same, it is prefer thatthe filled layers are thicker than the unfilled layers, i.e. the ratioof thickness of a filled layer to the thickness of an unfilled layer isgreater than 1:1. The ratio of the thickness of a filled layer to thethickness of an unfilled layer is suitably in the range from about 99:1to about 0.05:1, preferably about 50:1 to about 0.05:1 and is preferablyabout 10:1 to about 1:1.

Each of the two types of layer, i.e. the unfilled and filled layers, hasa tensile modulus which is within ±25%, preferably ±15%, more preferably±5% of the tensile modulus of the other type of layer. In oneembodiment, the two types of layer have substantially the same tensilemoduli. Preferably the polymeric material of each layer has a tensilemodulus of greater than 1380 MPa, preferably greater than 2000 MPa,preferably greater than 2070 MPa, preferably greater than 2760 MPa, andpreferably greater than 3000 MPa.

The respective layers of the multilayer film may be formed from anyfilm-forming material, particularly a polyester such as a syntheticlinear polyester which may be obtained by condensing one or moredicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters,eg terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6 or2,7-naphthalenedicarboxylic acid; succinic acid, sebacic acid, adipicacid, azelaic acid, 4,4′-diphenyldicarboxylic acid,hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly an aliphatic or cycloaliphatic glycol, egethylene glycol, 1,3-propanediol, 1,4butanediol, neopentyl glycol and1,4-cyclohexanedimethanol. An aliphatic glycol is preferred.Polyethylene terephthalate or polyethylene naphthalate is the preferredpolyester. Polyethylene terephthalate is particularly preferred.

In a preferred embodiment of the invention each type of layer of themultilayer film comprises the same material, preferably the samepolyester. Preferably, the materials comprising the various layers areprocessable at the same temperature and have similar melt viscosities soas to avoid degrading a lower melting material. Accordingly, residencetime and processing temperatures may have to be adjusted depending onthe characteristics of the materials of each layer. It is also preferredthat the layers comprise crystalline and/or semi-crystalline polyestermaterial.

Formation of a multilayer film may be effected by any of the techniquesknown in the art. Conveniently, however, formation of a composite filmis effected by coextrusion, either by simultaneous coextrusion of therespective film-forming layers through independent orifices of amulti-orifice die, and thereafter uniting the still molten layers, or,preferably, by single-channel coextrusion in which molten streams of thematerials of the respective film-forming layers are first united withina channel leading to a die manifold, and thereafter extruded togetherfrom the die orifice under conditions of streamline flow withoutintermixing.

One such technique is disclosed in U.S. Pat. No. 3,565,985 (Schrenk etal.). In making the multilayer films, melt coextrusion by either themulti-manifold die or the feedblock method in which individual layersmeet under laminar flow conditions to provide an integral multilayerfilm may be used. More specifically, separate streams of the materialsof each layer in a flowable state are each split into a predeterminednumber of smaller sub-streams. These smaller streams are then combinedin a predetermined pattern of layers to form an array of layers of thesematerials in a flowable state. Each layer is in intimate contact withadjacent layers in the array. This array generally comprises a tallstack of layers which is then compressed to reduce its height. In themulti-manifold die approach the film width remains constant duringcompression of the stack while the width is expanded in the feedblockapproach. In either case, a comparatively thin, wide film results. Layermultipliers in which the resulting film is split into a plurality ofindividual subfilms which are then stacked one upon another to increasethe number of layers in the ultimate film may also be used.

In one embodiment, the preparation of the multilayer film is effectedaccording to the procedure described in U.S. Pat. No. 3,051,453(re-issued as Re.28,072) and illustrated in FIG. 1. The patent reports aprocess in which a coextruded melt flow comprising three discrete layersfrom two polymers (BAB) is split and subsequently recombined as a stackof five distinct layers (BABAB). This procedure can be repeated to yielda single melt flow comprising higher orders of layer structure, whichmay then be extruded into a formed shape such as a film. Indeed, recentreports confirm that a symmetric multilayer structure can be producedafter as many as 7 layer multiplying units are incorporated in seriesinto a coextruded melt process (Nazarenko et al, SPE Inc., Tech. PapersVol 42, p1587, 1996).

The films described in the Examples hereinafter were prepared byfollowing the procedure described in U.S. Pat. No. 3,051,453. Thecoextruded melt comprising three layers (BAB) was processed througheither two or three layer multiplier units. The resulting multilayermelt was then directed through a slot die to be cast and quenched as athin film.

In a further embodiment, the preparation of the multilayer films may beeffected using layer multipliers on a composite stream comprisingdiscrete layers of coextruded polymeric material, as described below andin FIG. 2, wherein the z-axis is the direction of flow of a firstcomposite stream, the x-axis extends transversely of the first compositestream along a transverse dimension of the layer interface, and they-axis extends perpendicularly away from the layer interface in thedirection of the thickness of the layers of the first composite stream:

-   -   (1) expansion in the x-direction of a first composite stream        comprising coextruded, discrete layers of polymeric material,        wherein the interface between the layers lies in the x-z plane;    -   (2) division of the first composite stream along the x-axis into        multiple branch streams;    -   (3) re-orientation of the branch streams as they flow along the        x-axis so that they are stacked along the y-axis;    -   (4) recombination of the branch streams to form a second        composite stream; and    -   (5) contraction of the second composite stream along the y axis.

The layer-multiplication apparatus is positioned after the differentpolymers have been combined in a co-extrusion block. After the meltleaves the layer multiplier, it passes through a die and is thenmanufactured into a film with the desired crystallinity. The process maybe performed using any number of extruders to provide a first compositestream, as required. The first composite stream may also undergo layermultiplication using more than one layer multiplier, arranged in seriesor in parallel.

Other manufacturing techniques such as lamination, coating or extrusioncoating may be used in assembling multilayer films. For example, inlamination, a plurality of preformed layers having the requisitedifferences in optical properties are brought together under temperatureand/or pressure (e.g. using heated laminating rollers or a heated press)to adhere adjacent layers to each other. The films may also bemanufactured by successive casting of one or more layer(s) onto one ormore preformed layer(s). Extrusion coating may be preferred over themelt coextrusion process described above where it is desirable topretreat selected layers of the multilayer film or where the materialsare not readily coextrudable. In extrusion coating, a first layer isextruded onto either a cast web, a monoaxially oriented film or abiaxially oriented film and subsequent layers are sequentially coatedonto the previously provided layers. Exemplary of this method is U.S.Pat. No. 3,741,253.

In the manufacture of the multilayer films, any combination of the aboveprocess techniques may be adopted. For example, the use of more than twoextruders and the lamination of two or more co-extruded films may beused.

The layers of each type in the multilayer film may be uniaxiallyoriented, but are preferably biaxially oriented by drawing in twomutually perpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties.Orientation may be effected by any process known in the art forproducing an oriented film, for example a tabular or flat film process.

In a tubular process, simultaneous biaxial orientation may be effectedby extruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation.

In the preferred flat film process, the layer-forming polymer isextruded through a slot die and rapidly quenched upon a chilled castingdrum to ensure that the polymer is quenched to the amorphous stateOrientation is then effected by stretching the quenched extrudate in atleast one direction at a temperature above the glass transitiontemperature of the polymer. Sequential orientation may be effected bystretching a flat, quenched extrudate firstly in one direction, usuallythe longitudinal direction, i.e. the forward direction through the filmstretching machine, and then in the transverse direction. Forwardstretching of the extrudate is conveniently effected over a set ofrotating rolls or between two pairs of nip rolls, transverse stretchingthen being effected in a stenter apparatus. Stretching is effected to anextent determined by the nature of the polyester, for examplepolyethylene terephthalate is usually stretched so that the dimension ofthe oriented film is from 2 to 5, more preferably 2.5 to 4.5 times itsoriginal dimension in the or each direction of stretching. Greater drawratios (for example, up to about 8 times) may be used if orientation inonly one direction is required. It is not necessary to stretch equallyin the machine and transverse directions although this is preferred ifbalanced properties are desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional restraint at a temperature above theglass transition temperature of the polyester but below the meltingtemperature thereof, to induce crystallisation of the polyester. Inapplications where film shrinkage is not of significant concern, thefilm may be heat set at relatively low temperatures or not at all. Onthe other hand, as the temperature at which the film is heat set isincreased, the tear resistance of the film may change. Thus, the actualheat set temperature and time will vary depending on the composition ofthe film and its intended application but should not be selected so asto substantially degrade the tear resistant properties of the film.Within these constraints, a heat at temperature of about 135° to 235°C., preferably about 180 to 235° C., is generally desirable.

The opaque multi-layer film of the present invention is highly filled,preferably exhibiting a Transmission Optical Density (TOD) of at least0.25, more preferably at least 0.5, particularly at least 0.75, andideally at least 1.0. In one embodiment the TOD is in the range from 0.1to 3.0, more preferably 0.25 to 2.5, more preferably from 0.5 to 2.0,and particularly 0.75 to 1.25. The film is rendered opaque byincorporation into the polymer blend of the filled layer of an effectiveamount of an opacifying agent. Suitable opacifying agents include anincompatible resin filler, a particulate inorganic filler or a mixtureof two or more such fillers, as described hereinafter.

The surface of the opaque film preferably exhibits a whiteness index,measured as herein described, in the range from 60 to 120, morepreferably 80 to 110, particularly 90 to 105, and especially 95 to 100units.

Particularly improved aesthetic appearance occurs when the externalsurface of a film is matt, preferably exhibiting a 60° gloss value,measured as herein described, of less than 60%, more preferably in therange from 5% to 55%, particularly 20% to 50%, and especially 35% to45%.

By an “incompatible resin” is meant a resin which either does not melt,or which is substantially immiscible with the polymer, at the highesttemperature encountered during extrusion and fabrication of the film.The presence of an incompatible resin usually results in a voided layer,by which is meant that the layer comprises a cellular structurecontaining at least a proportion of discrete, closed cells. Suitableincompatible resins include polyamides and olefin polymers, particularlya homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbonatoms in its molecule. Preferred materials include a low or high densityolefin homopolymer, particularly polyethylene, polypropylene orpoly-4-methylpentene-1, an olefin copolymer, particularly anethylene-propylene copolymer, or a mixture of two or more thereof.Random, block or graft copolymers may be employed.

Particulate inorganic fillers include conventional inorganic fillers,and particularly metal or metalloid oxides, such as alumina, silica(especially precipitated or diatomaceous silica and silica gels) andtitania, calcined china clay and alkaline metal salts, such as thecarbonates and sulphates of calcium and barium. The particulateinorganic fillers may be of the voiding or non-voiding type. Suitableparticulate inorganic fillers may be homogeneous and consist essentiallyof a single filler material or compound, such as titanium dioxide orbarium sulphate alone. Alternatively, at least a proportion of thefiller may be heterogeneous, the primary filler material beingassociated with an additional modifying component. For example, theprimary filler particle may be treated with a surface modifier, such asa pigment, soap, surfactant coupling agent or other modifier to promoteor alter the degree to which the filler is compatible with thepolyester.

Preferred particulate inorganic, fillers include barium sulphate,titanium dioxide and silica, particularly barium sulphate arid titaniumdioxide.

The barium sulphate particles may be derived directly from the naturalore (Barites) or by synthetic precipitation (Handbook of Fillers forPlastics, H S Katz, J V Milewski. Van Nostrand Publishers, New York(1987)), for example by the precipitation reaction between barium saltsand sodium sulphate solutions. Synthetic BaSO₄ is generally of higherpurity than that derived from the natural ore, which itself isusually >98.5%. The barium sulphate should be finely divided. Barites iscommonly available with average diameter size of about 11 μm, 6 μm or 3μm. The average diameter of synthetic BaSO₄ is usually lower, withvalues commonly about 3 μm, 1 μm or 0.7 μm.

Titanium dioxide particles may be of anatase or rutile crystal form. Thetitanium dioxide particles preferably comprise a major portion ofrutile, more preferably at least 60% by weight, particularly at least80%, and especially approximately 100% by weight of rutile. Theparticles can be prepared by standard procedures, such as the chlorideprocess or the sulphate process. The titanium dioxide particles may becoated preferably with inorganic oxides such as aluminium, silcon, zinc,magnesium or mixtures thereof. Preferably the coating additionallycomprises organic compound(s), such as fatty acids and preferablyalkanols, suitably having from 8 to 30, preferably from 12 to 24 carbonatoms. Polydiorganosiloxanes or polyorganohydrogensiloxanes, such aspolydimethylsiloxane or polymethylhydrogensiloxane are suitable organiccompounds. The coating is suitably applied to the titanium dioxideparticles in aqueous suspension. The inorganic oxides are precipitatedin aqueous suspension from water-soluble compounds such as sodiumaluminate, aluminium sulphate, aluminium hydroxide, aluminium nitrate,silicic acid or sodium silicate. The coating layer on the titaniumdioxide particles is preferably in the range from 1 to 12% of inorganicoxides, and preferably in the range from 0.5 to 3% of organic compound,by weight based upon the weight of titanium dioxide.

The inorganic filler should be finely-divided, and the volumedistributed median particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value) thereof ispreferably in the range from 0.01 to 5 μm, more preferably 0.05 to 5 μm,more preferably 0.05 to 1.5 μm, more preferably 0.15 to 1.2 μm, andparticularly 0.15 to 0.5 μm.

Ideally, the largest particle should not have a size which is greaterthan the thickness of a filled layer. The presence of excessively largeparticles can result in the film exhibiting unsightly ‘speckle’, i.e.where the presence of individual filler particles in the film can bediscerned with the naked eye. Particles exceeding such a size may beremoved by sieving proceses which are known in the art. In a preferredembodiment, particle size is controlled as a function of the sizedistribution of the inorganic filler particles. Preferably, the sizedistribution is such that the Standard Deviation does not exceed 4 timesthe D(v,0.5) value, preferably 3 times the D(v,0.5) value and mostpreferably 2 times the D(v,0.5) value. As an illustration, wherein theinorganic filler particles have a measured D(v,0.5) value of 0.5 μmthen, on the basis that 99% of particles are within 3 standarddeviations of the D(v,0.5) value, it is preferred that 99% of the fillerparticles do not exceed 6.5 μm (this figure being calculated as 0.5μm+3×SD, where SD=4×0.5 μm), preferably that 99% of the filier particlesdo not exceed 5.0 μm (0.5 μm+3×SD, where SD=3×0.5 μm), and mostpreferably that 99% of the filler particles do not exceed 3.5 μm (0.5μm+3×SD, where SD=2×0.5 μm).

Particle size of the filler particles may be measured by electronmicroscope, coulter counter, sedimentation analysis and static ordynamic light scattering. Techniques based on laser light diffractionare preferred. The median particle sire may be determined by plotting acumulative distribution curve representing the percentage of particlevolume below chosen particle sizes aid measuring the 50th percentile.

The layers of the polyester film may, if desired, also contain any ofthe other additives conventionally employed in the manufacture ofpolymeric films. Thus, agents such as dyes, pigments, voiding agents,lubricants, anti-oxidants, anti-blocking agents, surface active agents,slip aids, gloss improvers, prodegradants, flame retardants,ultra-violet light stabilisers, viscosity modifiers and dispersionstabilisers may be incorporated as appropriate.

In a preferred embodiment, at least one and preferably at least theouter layers of the multilayer film comprise one or more UV absorber(s).The UV absorber(s) may be any known to those skilled in the art whichare compatible with the other materials used in the preparation of themultilayer films of the present invention.

In principle, any organic or inorganic UV absorber, particularly onewhich is suitable for use with polyester, may be employed in the presentinvention Suitable examples include the organic UV absorbes disclosed inEncyclopaedia of Chemical Technology, Kirk-Othmer, Third Edition, JohnWiley & Sons, Volume 23, Pages 615 to 627. Particular examples of UVabsorbers include benzophenones, benzotriazoles (U.S. Pat. Nos.4,684,679, 4,812,498 and 4,681,905), benzoxazinones (U.S. Pat. Nos.4,446,262, 5,251,064 and 5,264,539) and triazines (U.S. Pat. Nos.3,244,708, 3,843,371, 4,619,956, 5,288,778 and WO 94/05645). Theteaching of the aforementioned documents is incorporated herein byreference. Preferably, the UV absorber is non-volatile and does notcause excessive yellowing of the product.

In one embodiment of the invention, a UV absorber may be chemicallyincorporated in the chain of a layer-forming polyester. PreferredUV-stable polyesters arc produced by incorporating benzophenones intothe polyester, for example as described in EP-A-0006686, EP-A-0031202,EP-A-0031203 and EP-A-0076582, the teaching of which is incorporatedherein by reference.

In a preferred embodiment of the invention, the UV absorber comprisesone or more triazines, more preferably hydroxyphenyltriazines, andparticularly hydroxyphenyltriazine compounds of Formula 1:

wherein R is hydrogen, C₁-C₁₈ alkyl, C₂-C₆ alkyl substituted by halogenor by C₁-C₁₂ alkoxy, or is benzyl and R¹ is hydrogen or methyl. R ispreferably C₁-C₁₂ alkyl or benzyl, more preferably C₃-C₆ alkyl, andparticularly hexyl. R¹ is preferably hydrogen. An especially preferredUV absorber is 2-(4,6phenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxy-phenol,which is commercial available as Tinuvin™ 1577 FF from Ciba-Additives.Further examples of a preferred UV absorber are benzylidene malonateester (commercially available as Sanduvor™ PR-25 from Sandoz), andbenzoxazinone (commercially available as Cyasorb™ 3638 from Cytec).

Suitable inorganic UV absorbers include metal oxide particles, such aszinc oxide or titanium dioxide. Titanium dioxide particles, such asthose previously described herein, are particularly preferred.

The amount of UV absorber incorporated into a layer is generally in therange from 0.1% to 10%, more preferably 0.5% to 9%, more preferably 1.2%to 8%, particularly 2% to 6%, and especially 3.2% to 5.5% by weight,relative to the weight of the polymer of that layer. In one embodimentof the invention, both an organic UV absorber, preferably a triazine,and an inorganic UV absorber, preferably titanium dioxide, are present.The ratio, by weight of inorganic to organic UV absorber is preferablyin the range from 0.5 to 10:1, more preferably 1 to 5:1, andparticularly 1.5 to 2.5:1.

The components of the composition of a layer may be mixed together in aconventional manner. For example, by mixing with the monomeric reactantsfrom which the layer polymer is derive or the components may be mixedwith the polymer by tumble or dry blending or by compounding in anextruder, followed by cooling and, usually, comminution into granules orchips. Masterbatching technology may also be employed.

The presence of the particulate filler generally also improves thewindability of the film during manufacture. However, to increasewindability, the film may also comprise a “slip coating”. A suitableslip coating may be, for instance a discontinuous layer of an acrylicand/or methacrylic polymeric resin optionally further comprise across-linking agent, such as described in EP-A0408197, the disclosure ofwhich is incorporated herein by reference. An alternative slip coatingmay comprise a potassium silicate coating, for instance as disclosed inU.S. Pat. Nos. 5,925,428 and 5,882,798, the disclosures of which areincorporated herein by reference.

The multilayer films may also comprise an ink-receptive coating. Theink-receptive coating improves the adhesion of ink to the film andincreases the range of inks that can be readily applied to the surface.The ink-receptive coating may be any such coating well-known to thoseskilled in the art. For example, the ink-receptive coating may comprisean acrylic component and a cross-linking component (e.g. melamineformaldehyde), such as the coating disclosed in EP-A-0429179, thedisclosure of which is incorporated herein be reference.

According to a further aspect of the invention there is provided aprocess for the manufacture of an opaque, tear-resistant film, saidprocess comprising the steps of providing a plurality of layers ofpolymeric material, wherein said plurality of layers comprises at leasttwo layers of filled polymeric material containing therein at least 5%by weight of opacifying agent and at least two layers of unfilledpolyester material substantially devoid of opacifying agent, and forminga composite film of said plurality of layers.

According to a further aspect of the present invention, there isprovided the use of two or more layers of unfilled polymeric materialsubstantially devoid of opacifying agent to improve the tear-resistanceof an opaque multilayer film comprising at least two filled layers ofpolymeric material wherein said filled layers comprise at least 5% byweight of opacifying agent.

According to a further aspect of the present invention, there isprovided a method for improving the tear-resistance of an opaquemultilayer polymeric film comprising at least two filled layers ofpolymeric material wherein said filled layers comprise at least 5% byweight of opacifying agent, said method comprising incorporating intothe film at least two unfilled layers of polymeric materialsubstantially devoid of opacifying agent.

The opaque polymeric films of the present invention have a wide varietyof uses including use generally as a paper replacement (such as in filmsfor graphics, displays, labels, cards (including identity cards, smartcards and credit cards) and imaging), as printing plate media, inpackaging and in coil coating. The film would also be beneficial forlaminating to the internal walls of a metal can, particularly a tin can.The films are especially useful in applications requiring thinner gaugesof opaque tear-resistant film, particularly labels and packagingapplications. As noted above, a thinner gauge can offer cost savings toprice-sensitive markets and this is particularly important in packagingapplications. The films are also especially useful in applicationsdemanding a film product having a high tear-resistance.

The invention is illustrated by reference to the following figures inwhich:

FIG. 1 is a schematic illustration of a layer-multiplying arrangement asdisclosed in U.S. Pat. No. 3,051,453 which was used to manufacture themultilayer films. The 3-layer melt stream enters the apparatus from theleft hand side and exits the apparatus at the right hand side as shownin FIG. 1. Cross-sections of the melt stream are illustrated as inserts(a) to (e). The position of the dashed line A-A in insert (a) is shownin the schematic illustration of the apparatus. Similar sections areshown as B-B, C-C and D-D for inserts (b), (c) and (d) respectively.Thus, inserts (a) to (e) illustrate cross-sectional views of thestructure of the multilayer, coextruded melt flow at various stagesduring the layer multiplying treatment. Referring to insert (a), a threelayer composite melt stream, which is to be layer-multiplied, is presentas shown before admission to the first multiplying device. The meltstream is then successively divided, compressed and recombined at stages(a), (b) and (c) respectively. A repeat treatment is illustrated, whichyields a nine layer composite stream which is then transported to thedie. A further repeat treatment would yield a structure comprising 17layers.

FIG. 2 is a schematic illustration of a layer-multiplying arrangementwhich may be used to manufacture the multilayer films.

Referring to FIG. 2, a two layer composite melt steam (1) is passed to acoextrusion block (2) and then passed through a four-lane layermultiplier (3). The polymeric material is then passed to the die (4) toprovide an eight layer composite stream (5).

FIGS. 3B to 3D and FIGS. 4A to 4D are graphical representations of thetest data obtained by analysing the films made as described in thefollowing Examples. Further discussion of FIGS. 3B to 3D and 4A to 4D isprovided below.

The following test methods may be used to determine certain propertiesof the film:

-   -   (i) Transmission Optical Density (TOD) of the film is measured        in accordance with ASTM D1003-97 using a BYK Gardner Inc.        Hazegard System in transmission mode.    -   (ii) L*, a* and b* colour co-ordinate values (CIE (1976)) and        whiteness index of the external surface of the white layer are        measured using a Colorgard System 2000. Model/45 (manufactured        by Pacific Scientific) based on the principles described in ASTM        D313.    -   (iii) 60° gloss value of the film surface is measured using a Dr        Lange Reflectometer REFO 3 (obtained from Dr Bruno Lange, GmbH,        Dusseldorf, Germany) according to DIN 67530.    -   (iv) The maximum load and tear toughness of the film in both the        machine and transverse directions were measured using the Graves        Tear Test (ASTM D1004-94A) and as described herein.    -   (v) Tensile modulus and tensile elongation may be measured in        accordance with ASTM D882-88.

The invention is further illustrated by reference to the followingexamples.

EXAMPLES

The films were manufactured using a standard flat film extrusion andstenter process with the following materials:

-   -   Polymer “A”: Poly(ethylene terephthalate) (PET) having an        intrinsic viscosity (IV) of 0.63±0.02    -   Polymer “B”: PET containing 20 w/w % banum sulphate filler        having an IV of 0.63±0.02    -   Blend 1: 100% Polymer B    -   Blend 2: 80% Polymer B; 20% Polymer A    -   Blend 3: 70% Polymer B; 30% Polymer A    -   Blend 4: 60% Polymer B; 40% Polymer A    -   Blend 5: 50% Polymer B; 50% Polymer A

Polymers A and B were used to make the unfilled and filled layers of amultilayer film, respectively. Blends 1 to 5 were used to make monolayerfilms prepared for the purposes of comparison with the multila yerfilms.

A melt coexusion system was used in which two separate extruderssupplied polymer in the molten state through a coextrusion injectorblock to a common melt channel. The feature of the injection block isthat the two polymer melt flows are introduced to, and remain discretelayers in, a common melt pipe.

The coextruding melt was passed through a system capable of splittingand recombining the flow such that the number of discrete layers ofpolymer melt was increased within the melt channel. The method toproduce multiple layers of polymer from an initial flow consisting of 3layers was similar to techniques described in U.S. Pat. No. 3,051,453,discussed above. Where identical polymer was fed by each extruder intothe melt system for the purpose of providing Comparative Examples, asingle or monolayer of polymer was processed.

The molten polymer was subsequently extruded through a flat film die,and cast as a melt curtain onto a cold, rotating chill roll. Thereafterthe cast film was subjected to several further stages of processing. Thefilm was fed through a stretching stage, where a draw ratio between 2.9and 3.1 in the forward or machine direction at 90° C. was imposed on thematerial. A sideways or transverse draw of ratio 3.2 was then applied bya tentering method at a temperature of 110° C., and finally thebiaxially-oriented film was crystallised in a heat-set stage attemperatures around 220° C.

Table 1 shows the process conditions of film manufacture and theproperties of the films produced. In order to assess the characteristicsof the multilayer films of the invention, a series of monolayer films ofcorresponding thicks was also prepared for comparison purposes. The datafor the optical density and maximum load have been normalised to an 80μm film.

The data for optical density and tear-resistance shown in Table 1 arealso represented in FIGS. 3B to 3D and in FIG. 4A to 4D. In FIGS. 3B to3D the optical density is plotted as a function of filler concentration.In FIGS. 4A to 4D the tear-toughness is plotted as a function of fillerconcentration.

FIG. 4A displays the tear toughness data for the multilayer films ofComparative Examples 1 to 4 and the data for the monolayer ComparativeExamples 5 to 7. FIGS. 3B and 4B display the opacity and tear toughnessdata for Examples 1 to 5 (9 layers) and monolayer Comparative Examples 8to 11.

FIGS. 3C and 4C display the opacity and tear toughness data for Examples6 to 9 (17 layers) and monolayer Comparative Examples 12 to 14.

FIGS. 3D and 4D display the opacity and tear toughness data for Examples10 to 13 (17 layers) and monolayer Comparative Examples 15 to 17.

FIGS. 3B to 3D demonstrate that for a given loading of filler, amultilayer film according to the invention exhibits improved opticaldensity in relation to a monolayer film having the corresponding loadingof filler.

FIGS. 4B to 4D demonstrate that for a given loading of filler, amultilayer film according to the invention unexpectedly exhibits agreater tear toughness in relation to a monolayer film having thecorresponding loading of filler. FIG. 4A demonstrates that this effectis not shown for a three layer film, which performs in a similar mannerto the monolayer film.

Other advantages of the films of the present invention are apparent byfurther analysis of this data.

It is clear from FIG. 3B that in order to match the opacity of the mosthighly filled mono-layer film, a multilayer film must exhibit an opticaldensity of about 1.02. The filler loading required to achieve this in amonolayer film is around 1997, whereas for a multilayer film accordingto the invention this optical density can be achieved using only about16% filler.

Thus, for a given optical density, the multilayer films of the presentinvention are more economical to produce than the monolayer films of theprior art.

Moreover, when one compares the tear toughness of these two films havingmatching opacity (see FIG. 4B), the multilayer film (approximately 16%filler) exhibits a tear toughness of approximately 0.42 kg/mm², which isalmost double that of the equivalent-opacity monolayer film(approximately 19% filler) which has a tear toughness of 0.26 kg/mm².TABLE 1 Graves Tear Test No. Feed Rate* Draw Film Total OpticalTear-Toughness of Film structure Stream 1 Stream 2 Ratio thicknessfiller Density Max Load (kgf) (kg/mm²) Film layers and Composition Kg/hrkg/hr (FD × SD) (μm) (%) (normalised) MD TD MD TD C. Ex 1 3 BAB 93 402.9 × 3.2 81 12.9 — 3.52 — 0.466 — C. Ex 2 3 BAB 72 48 2.9 × 3.2 76 11.3— 3.79 — 0.497 — C. Ex 3 3 BAB 60 60 2.9 × 3.2 86 9.6 — 3.85 — 0.611 —C. Ex 4 3 BAB 50 75 2.9 × 3.2 92 7.9 — 3.99 — 0.657 — C. Ex 5 1 Mono;Blend 5 72 48 2.9 × 3.2 61 10.5 — 3.94 — 0.542 — C. Ex 6 1 Mono; Blend 272 48 2.9 × 3.2 90 16.2 — 3.76 — 0.411 — C. Ex 7 1 Mono; Blend 1 72 482.9 × 3.2 76 19.6 —/ 3.11 — 0.24 — Ex. 1 9 (BA)₄B 149 40 3.1 × 3.2 79.916.1 1.05 — — — — Ex. 2 9 (BA)₄B 93 40 3.1 × 3.2 75.7 15.7 0.95 3.602 —0.418 — Ex. 3 9 (BA)₄B 71 48 3.1 × 3.2 78.8 13.1 0.85 4.019 — 0.615 —Ex. 4 9 (BA)₄B 59 60 3.1 × 3.2 85.5 11.1 0.76 3.873 — 0.687 — Ex. 5 9(BA)₄B 50 75 3.1 × 3.2 75.1 8.6 0.75 3.937 — 0.937 — C. Ex. 8 1 Mono;Blend 5 72 48 3.1 × 3.2 76.9 9.4 0.55 3.661 — 0.536 — C. Ex. 9 1 Mono:Blend 4 72 48 3.1 × 3.2 75.4 11.1 0.65 3.522 — 0.494 — C. Ex. 10 1 Mono:Blend 2 72 48 3.1 × 3.2 81.1 14.7 0.71 3.399 — 0.382 — C. Ex. 11 1 Mono;Blend 1 72 48 3.1 × 3.2 81.1 18.7 1.02 3.264 — 0.262 — Ex. 6 17 (BA)₈B109 50 3.1 × 3.2 95.33 19.3 0.99 3.022 2.769 0.257 0.291 Ex. 7 17 (BA)₈B90 56 3.1 × 3.2 81.3 17.9 1.07 3.354 3.091 0.294 0.321 Ex. 8 17 (BA)₈B80 60 3.1 × 3.2 100.8 17.2 0.88 2.935 2.743 0.353 0.35 Ex. 9 17 (BA)₈B70 65 3.1 × 3.2 83.7 11.6 0.86 3.329 3.122 0.508 0.401 C. Ex. 12 1 Mono:Blend 5 72 48 3.1 × 3.2 79.7 10.9 0.62 3.537 3.459 0.344 0.377 C. Ex. 131 Mono: Blend 3 72 48 3.1 × 3.2 87.8 15.4 0.86 3.248 2.837 0.244 0.25 C.Ex. 14 1 Mono; Blend 1 72 48 3.1 × 3.2 81.3 21.4 1.13 2.872 2.517 0.1770.179 Ex. 10 17 (BA)₈B 109 50 2.9 × 3.2 93.2 19.3 0.90 3.588 3.285 0.3110.329 Ex. 11 17 (BA)₈B 90 56 2.9 × 3.2 83.7 17.9 0.96 3.558 3.294 0.3270.339 Ex. 12 17 (BA(₈B 80 60 2.9 × 3.2 107.1 17.2 0.77 3.838 3.273 0.4350.402 Ex. 13 17 (BA)₈B 70 65 2.9 × 3.2 87.8 11.6 0.77 3.688 3.366 0.6620.57 C. Ex 15 1 Mono; Blend 5 72 48 2.9 × 3.2 81.9 10.9 0.49 4.053 3.5350.538 0.419 C. Ex 16 1 Mono; Blend 3 72 48 2.9 × 3.2 86.5 15.4 0.743.794 3.413 0.36 0.296 C. Ex 17 1 Mono; Blend 1 72 48 2.9 × 3.2 76.221.4 1.07 3.521 3.205 0.237 0.208*Stream 1 and Stream 2 carry polymers A and B, resepctively. Where amono-layer film is produced, Streams 1 and 2 carry the same polymer.

1. An opaque multilayer film having a tear toughness in at least onedimension of the film of at least 0.3 kg/mm² measured as the areabeneath the curve in a graphical plot of stress versus strain inaccordance with ASTM D1004-94A, and comprising at least two unfilledpolyester layers substantially devoid of opacifying agent and at leasttwo filled layers of polymeric material wherein said filled layerscomprise at least 5% by weight of opacifying agent, wherein the filledand unfilled layers alternate.
 2. A film according to claim 1, whereinthe polymeric material of said filled layers is polyester.
 3. A filmaccording to claim 1 wherein the polymeric material of the layerssubstantially devoid of opacifying agent is the same as that of thepolymeric material of the layers comprising at least 5% by weight ofopacifying agent.
 4. A film according to claim 1 wherein the polymericmaterial of a layer is poly(ethylene terephthalate).
 5. A film accordingto claim 1 wherein said opacifying agent is an inorganic particulatefiller.
 6. A film according to claim 1 wherein said opacifying agent isbarium sulphate.
 7. A film according to claim 1 wherein said filledlayers comprise opacifying agent in the range of from 10 to 30% byweight of the polymeric material of the filled layer.
 8. A filmaccording to claim 1 having a multilayer structure (BA)_(n)B wherein Arepresents an unfilled layer, B represents a filled layer and n is atleast
 2. 9. A film according to claim 8 wherein n is at least
 3. 10. Afilm according to claim 1 wherein the total number of layers is 7 ormore.
 11. A film according to claim 1 having a transmission opticaldensity of at least 0.5.
 12. A process for the manufacture of an opaque,tear-resistant film having a tear toughness in at least one dimension ofthe film of at least 0.3 kg/mm² measured as the area beneath the curvein a graphical plot of stress versus strain in accordance with ASTMD1004-94A, said process comprising the steps of providing a plurality oflayers of polymeric material, wherein said plurality of layers comprisesat least two layers of filled polymeric material containing therein atleast 5% by weight of opacifying agent and at least two layers ofunfilled polyester material substantially devoid of opacifying agent,and forming a composite film of said plurality of layers wherein thefilled and unfilled layers alternate.
 13. The use of a multilayer filmstructure comprising at least two unfilled layers of polymeric materialsubstantially devoid of opacifying agent and at least two filled layersof polymeric material wherein said filled layers comprise at least 5% byweight of opacifying agent for providing improved tear-resistance in anopaque polymeric film such that said film has a tear toughness in atleast one dimension of the film of at least 0.3 kg/mm² measured as thearea beneath the curve in a graphical plot of stress versus strain inaccordance with ASTM D1004-94A.
 14. A method for improving thetear-resistance of an opaque multilayer polymeric film comprising atleast two filled layers of polymeric material wherein said filled layerscomprise at least 5% by weight of opacifying agent, said methodcomprising incorporating into the film at least two unfilled layers ofpolymeric material substantially devoid of opacifying agent, such thatsaid film has a tear toughness in at least one dimension of the film ofat least 0.3 kg/mm² measured as the area beneath the curve in agraphical plot of stress versus strain in accordance with ASTMD1004-94A.
 15. The use of two or more layers of unfilled polymericmaterial substantially devoid of opacifying agent to improve thetear-resistance of an opaque multilayer film comprising at least twofilled layers of polymeric material wherein said filled layers compriseat least 5% by weight of opacifying agent, such that said film has atear toughness in at least one dimension of the film of at least 0.3kg/mm² measured as the area beneath the curve in a graphical plot ofstress versus strain in accordance with ASTM D1004-94A.
 16. (Cancelled).